1. Field of the Invention
This invention relates to tomography and, more particularly, to tomography wherein an image of an object is directly reconstructed with sub-wavelength resolution.
2. Description of the Background Art
Total internal reflection microscopy (TIRM) brings to conventional microscopy the added functionality of illumination by evanescent waves. Incorporation of evanescent waves in the illuminating field is an important development for several reasons. First, the exponential decay of such waves along one direction allows for the control of the depth of penetration of the illuminating field. Second, evanescent waves may be used to resonantly excite surface plasmon modes of the sample. Finally, and perhaps most importantly, evanescent waves may be employed to supercede the Rayleigh diffraction limit of order half the wavelength, xcex/2.
TIRM has been in practical use for decades. It has primarily been used as a surface inspection technique, though the sensitivity of the field to distance along the decay axis has been utilized to advantage in applications such as the measurement of distance between two surfaces. Until recently the opportunities for transverse superresolution made possible by the high spatial frequency content of the probe field have been largely over-looked. However, there have been advances in the realization of super-resolved imaging utilizing TIRM. There had been an initial direct imaging approach resulting from the marriage of standing-wave illumination techniques and TIRM achieving transverse resolution of xcex/7. A later approach in which the information content of the TIRM experiment for the case of weakly-scattered fields has been determined is reported by Fischer in xe2x80x9cThe information content of weakly scattered fields: implications for near-field imaging of three-dimensional structuresxe2x80x9d, published in Journal of Modern Optics, Vol. 47, No. 8, pages 1359-1374, 2000.
However, the art does not teach or suggest a technique for the reconstruction of a tomographic image of the object which is accomplished by making use of an analytic solution to an inverse scattering formulation with evanescent waves.
These shortcomings, as well as other limitations and deficiencies, are obviated in accordance with the present invention via a technique, referred to as total internal reflection tomography (TIRT,) whereby an object is probed with evanesent waves and then waves scattered from the object are processed with a prescribed mathematical algorithm to reconstruct the tomographic image.
In accordance with a broad method aspect of the present invention, a method for generating a tomographic image of an object includes: (1) probing the object with incident evanescent waves; (2) detecting waves scattered by the object, and (3) reconstructing the tomographic image of the object by executing a prescribed mathematical algorithm with reference to the incident evanescent waves and the scattered waves to generate the tomographic image with subwavelength spatial resolution.
A broad system aspect of the present invention is commensurate with the broad method aspect.
The features of the TIRT approach are at least three-fold: (i) the evanescent waves used for illumination encode on the scattered field the subwavelength structure of the scattering object. It is thus possible to obtain subwavelength resolved images of the sample as is done in other near-field techniques such as near-field scanning optical microscopy without the technical difficulties encountered with probe-sample interactions; and (ii) the results of the reconstruction are unambiguous in the sense that the relation between the scattered field and the three-dimensional structure of the sample, as described by the spatial dependence of the susceptibility, is made manifest. This is somewhat analogous to the transition from projection radiography to computed tomography.